Method of making a polarization-insensitive, evanescent-wave, fused coupler with minimal environmental sensitivity

ABSTRACT

An optical coupler and method of making same is described. The coupling ratio of the coupler is polarization-insensitive. The optical coupler described herein is made from single-mode optical fibers. Each optical fiber has a length of nearly exposed core which is fused to the exposed core of the other optical fiber while the fibers are maintained in parallel juxtaposition with one another without twisting. By creating a fused core coupler from single-mode optical fibers in which the cores are in parallel juxtaposition with one another, the coupling ratio of the subject invention does not change with changes in polarization of light passing through each single-mode fiber and, thus, is polarization insensitive.

This is a continuation of co-pending application Ser. No. 498,436 filedon May 26, 1983, now abandoned.

BACKGROUND OF THE INVENTION

A. Field of the Invention

The present invention relates generally to optical waveguide couplersand more specifically to optical couplers using single-mode opticalfibers that are resistant to polarization changes.

B. Description of the Prior Art

Fiber optic couplers are commonly used to transfer light from oneoptical waveguide to another for a variety of functions. For example,fiber optic couplers are used in the communications field to rapidlyconvey .[.larger.]. .Iadd.large .Iaddend.amounts of data. Otherapplications include measurement systems in which high-precisionmeasurements are made based upon an environmentally dependent shift ineither the phase or wavelength of light traveling through an opticalwaveguide. Early couplers used for measurement applications had severaldisadvantages. For instance, the coupling ratio of early couplerstypically varied with the polarization of the light passing through thecoupler. Furthermore in some cases, the phase of the light passingthrough the coupler varied with polarization which was unsatisfactoryfor certain interferometric sensor .[.aplications.]. .Iadd.applications.Iaddend.such as the passive quadrature demodulator coupler matrix.

Much of the technology developed in the past ten years for manufacturingcouplers has centered around couplers which use multimode fibers.Multimode optical fibers are fairly large fibers, having a core diameteron the order of 40 microns to 200 microns. Some multimode fibers may beas large as 4 millimeters. While multimode fibers can be used to producecouplers for transferring polarized light from one fiber to another,they are typically undesirable because polarized light transmitted in amultimode optical fiber has a tendency to become depolarized.Single-mode fibers, on the other hand, are more capable of transmittingpolarized light with minimal depolarization of light during transmissionover small distances. Due to the method in which single-mode fibers actto transmit only one mode of light, the core diameter of a single-modefiber is approximately ten times smaller than the core diameter of amultimode fiber. This had led to serious difficulties in manufacturingcouplers using single-mode fibers. The invention described hereinbelowrepresents an advancement in the art in that it teaches methods forcoupling single-mode fibers and for producing polarization-insensitiveoptical couplers.

Before describing the applicants' invention however, it is necessary tobriefly describe the physical characteristics of a typical opticalfiber. In most instances, a fiber comprises an inner core having arelatively high index of refraction, a cladding having a relatively lowindex of refraction, and a substrate having a relatively high index ofrefraction, but not necessarily equal to the index of refraction of thecore. This type of fiber is commonly called "W-fiber" by those skilledin the art becamse the index profile of the fiber appears to resemble a"W". It should be noted that other types of fibers do not have an outersubstrate having a relatively high index of refraction.

SUMMARY OF THE INVENTION

The subject development comprehends a polarization-insensitive opticaldevice for coupling light which includes a first single-mode opticalfiber having a first longitudinal segment having an inner core having arelatively high index and a cladding having a relatively low index ofrefraction. The optical device also includes a second single-modeoptical fiber having a second longitudinal segment having an inner corehaving a relatively high index of refraction and a cladding having arelatively low index of refraction. A coupling region is included in thesubject device having the first and second longitudinal portions fusedtogether in parallel juxtaposition to one another. The coupler furtherincludes a housing means for maintaining the first and secondlongitudinal portions substantially straight and for maintaining thefirst and second longitudinal portions in a stable environment.

The invention further comprehends a method of makingpolarization-insensitive optical couplers comprising the steps ofexposing first and second longitudinal cladding segments of first andsecond single-mode optical fibers; maintaining the optical fibers inlinear parallel juxtaposition with one another along a portion of theexposed longitudinal segments; fusing the parallel juxtapositionsegments of the fibers together to form a coupling region, and rigidlyencapsulating the coupling region of the optical fibers whilemaintaining the linear parallel juxtaposition of the fibers.

In view of the preceding, an object of the invention described herein isto provide a polarization-insensitive coupler which is simple tofabricate using single-mode optical fibers.

It is a further object of the invention to provide an environmentallystable single-mode optical fiber.

It is a further object of the invention to provide a single-mode couplerdevice having controlled coupling.

These objects are given only by way of example; thus, other objectivesand advantages inherently achieved by the disclosed invention may occurto those skilled in the art. The scope of the invention is to be limitedonly by the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a frontal view of one embodiment of the coupler describedherein indicating the coupling region;

FIG. 2 is a frontal view of one embodiment of the coupler describedherein illustrating a quartz housing in which the housing is broken awayto provide a view of the coupling region;

FIG. 3 is an illustration partially in phantom of one embodiment of thesubject coupler after encapsulation; and

FIG. 4 is an isometric illustration of one embodiment of the equipmentused to fabricate the couplers described herein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As discussed above, single-mode fibers are extremely fragile because oftheir minute size. The subject development, therefore, has been directedto providing a single-mode coupler which is relatively rugged, and isrelatively insensitive, or stable, with respect to changes inpolarization. Referring now to FIG. 1, first and second single-modeoptical fibers 12, 14 are provided in the preferred embodiment of thesubject development. Each optical fiber 12, 14 has a respectivesubstrate 16, 18 and core and cladding 20, 22. The diameter of the coreof each single-mode fiber is on the order of 5 microns whereas thediameter of the cladding of each fiber is on the order of 75-125microns. Thus, it should be recognized that when the core 20, 22 of eachfiber is exposed, it is in a relatively delicate, or fragile, state. Inthe preferred embodiment of the subject development, each fiber 12, 14has a longitudinal segment having a core and cladding diameter 20, 22 ofapproximately 15-50 microns. It should be obvious to one skilled in theart that the length of the longitudinal segment can be varied greatlywithout deviating from the intent of the subject invention. In thepreferred embodiment, the exposed core length is approximately onecentimeter in order to ensure complete coupling between the firstoptical fiber 12 and the second optical fiber 14. The longitudinalsegments 20, 22 are fused together in coupling region 24. The inventiondescribed herein comprehends that throughout the coupling region, thefirst and second fibers 12 and 14 have cores which are maintained inlinear parallel juxtaposition with one another.

Referring now to FIG. 2, the fused fiber 12, 14 of FIG. 1 areillustrated in a rigid housing 26. The method of placing the fusedfibers within the housing will be discussed in greater detailhereinbelow. It is important to note, at this point however, that thehousing provides a rigid support to fused fibers 12 and 14 throughoutcoupling region 24.

Referring now briefly to FIG. 3, after fibers 12 and 14 have beenpositioned in housing 26, the entire assembly is encapsulated in arubber-like material 28. Encapsulation of the assembly provides afurther means for stabilizing the subject device. The encapsulationmaterial may provide strain relief to the ends of the fibers whichextend beyond the housing because the housing may be encapsulated by adipping technique in which the ends of the fibers are also coated withthe encapsulation material.

The following description represents the currently preferred method ofmaking the subject couplers. In the preferred embodiment, twosingle-mode fibers are prepared. A variety of single-mode fibers may beused, depending on the wavelength of operation of the coupler. Eachcoupler would be designed to be laser specific, that is, the fiber usedfor one wavelength of light may be made from a different material, andhave a different core diameter and index of refraction than a couplerdesigned for another wavelength of operation. Important characteristicsof any fiber chosen include: good concentricity, excellent coreuniformity, and an appropriate refractive index profile. While variouslengths of optical fiber may be used, the present procedure in thelaboratory is to use two lengths of optical fiber that are 50centimeters long. Each fiber has a plastic jacket and an RTV (roomtemperature vulcanizing .[.silicone.]. .Iadd.silicon.Iaddend.) coating.The plastic jacket is cut away and any RTV is removed with ahydrofluoric acid rinse.

Referring now to FIG. 4, after the fibers have been prepared asdiscussed above, each fiber 12 and 14 is placed in respective groves30-33 of clamping devices 36, 38. The fibers are then cleaned with ahydrofluoric acid/water/alcohol rinse according to techniques known tothose skilled in the art. After each fiber has been inspected forcleanliness, etching of the fibers is initiated. It is the object of theetching portion of the procedure to remove the substrate 16 and 18 ofeach fiber 12 and 14 (FIG. 1) to expose core and cladding 20 and 22along a coupling region 24 of each of the fibers so as to provide anappropriate index profile. An appropriate index profile is one in whichthe outer surface, or cladding, of each fiber has a lower index ofrefraction than the core of each fiber. Although a variety of etchingtechniques may be used, in view of the particular fiber used in thepreferred embodiment, it is desirable to etch the subject fibers by aheated etching technique. In this technique, the fibers are placed inclose proximity to an etching station which is heated by athermoelectric module. A drop of etchant is placd on top of the etchingstation to etch a longitudinal portion of the fiber. After the fiber hasbeen etched to the desired diameter, the fibers are then rinsed withwater to prevent further etching. The very fragile etched longitudinalsegments 20, 22 are then brought in parallel juxtaposition with oneanother by wrapping opposite ends of each exposed core length to oneanother with a thread or other wrapping material 42, 44. In thepreferred embodiment, it has been found that a thin plastic sheet isdesirable to prevent breakage of the fragile cores 20, 22. After thecores have been temporarily wrapped as discussed above to provide thedesired parallel juxtaposition special relationship, a small drop ofadhesive material is deposited on each end 46, 48 of the exposed corelength adjacent to threads 42, 44. The glue is allowed to cure so thatthe cores 20, 22 remain in parallel juxtaposition with one anotherthroughout coupling region 24 when threads 42 and 44 are removed fromthe exposed cores 20, 22.

The coupling region of the fibers 12, 14 are then heated. In thelaboratory technique used in the preferred embodiment, a lighted torchis simply passed along the coupling region 24 while the longitudinalsegments are in axial tension until the segments 20, 22 are fusedtogether throughout the length of the coupling region. A quartz tube 26is then measured and cut to the desired length to surround the exposedcores of the subject coupler. In one embodiment, a slot extending thelength of the quartz tube 26 may be provided to allow insertion of thetube about fibers 12, 14. In another embodiment, the tube may be cut inhalf longitudinally to allow each half to be simply placed about thefibers. The ends of the fibers 12, 14 are then glued to the ends 81, 83of the quartz tube to suspend the etched and fused segments of thefibers containing the coupling region 24 in the approximate center ofthe tube 26. The glued assembly is then allowed to cure as necessary.The quartz tube is then dipped in a material to provide an elasticcovering over tube 26.

The invention described herein is insensitive to changes in polarizationdue to its unique design. In other single-mode, evanescent-wavecouplers, birefringence resulting from twists, bends, stresses, orstrains imposed on the fibers will cause changes in polarity of lighttransmitted through the fibers. It is common practice in couplermanufacture to twist a pair of fibers about one another in order tomaintain contact during fusing. By eliminating the need to twist thefibers about one another and by making bends in the fiber as gradual aspossible, the polarization dependence of the coupling ratio iseliminated.

Although the invention has been described and illustrated in detail, itis to be clearly understood that the same is by way of illustration anexample only, and is not to be taken by way of limitation; the spiritand scope of this invention being limited only by the terms of theappended claims.

We claim:
 1. A method of making a polarization-insensitive opticalcoupler comprising the steps of:(a) etching first and secondlongitudinal segments of first and second single-mode optical fibers sothat each fiber has an outer cladding having a low index of refractionrelative to the index of refraction of a nearly exposed core of each ofthe first and second single-mode optical fibers; (b) maintaining thefirst and second single-mode optical fibers in parallel juxtapositionwith one another along a portion of each of the first and secondlongitudinal .[.segment.]. .Iadd.segments.Iaddend., said maintainingstep comprising the substeps of:(b1) wrapping portions of the first andsecond longitudinal segments with a material which will notsubstantially deform the nearly exposed cores of the first and secondsingle-mode optical fibers; (b2) gluing the nearly exposed cores of thefirst and second single-mode optical fibers together with glue while thenearly exposed cores are wrapped; (b3) curing the glue; and (b4)unwrapping the nearly exposed cores; (c) fusing together the portions ofthe first and second longitudinal segments maintained in paralleljuxtaposition to form a coupling region; and (d) rigidly encapsulatingthe nearly exposed cores of the first and second single-mode opticalfibers while maintaining the parallel juxtaposition of the first andsecond longitudinal segments.
 2. A method of making apolarization-insensitive optical couopler as recited in claim 1, whereinsaid step (a) comprises etching the first and second longitudinalsegments along a length in the range of 0.5 to 2 cm.
 3. A method ofmaking a polarization-insensitive optical coupler as recited in claim 1,wherein said step (a) includes removing an outer jacket of each of thefirst and second single-mode optical fibers along the length of thenearly exposed cores of the first and second single-mode optical fibers.4. A method of making a polarization-insensitive optical coupler asrecited in claim 1, wherein said step (a) includes rinsing the first andsecond longitudinal segments of the first and second single-mode opticalfibers in hydrofluoric acid to remove any room temperature vulcanizingrubber which may be surrounding the cladding of the first and secondsingle-mode optical fibers.
 5. A method of making apolarization-insensitive optical coupler as recited in claim 1, whereinsaid step (b) includes aligning the first and second single-mode opticalfibers in a holder having alignment grooves prior to said substep (b1).6. A method of making a polarization-insensitive optical coupler asrecited in claim 1, wherein said step (d) includes the substeps of:(d1)placing a tube having ends and having a length substantially the same asthe first and second longitudinal segments over the coupling region;(d2) gluing the ends of the tube to the first and second single-modeoptical fibers with glue to suspend the fibers within the tube; and (d3)curing the glue.
 7. A method of making a polarization-insensitiveoptical coupler comprising the steps of:(a) etching first and secondlongitudinal segments of first and second single-mode optical fibers sothat each fiber has an outer cladding .[.have.]. .Iadd.having .Iaddend.alow index of refraction relative to the index of refraction of a nearlyexposed core of each of the first and second single-mode optical fibers;(b) maintaining the first and second single-mode optical fibers inparallel juxtaposition with one another along a portion of each of thefirst and second longitudinal segments while placing the first andsecond longitudinal segments under axial tension; (c) fusing theparallel juxtaposition portions of the first and second longitudinalsegments together while the first and second longitudinal segments aremaintained in parallel juxtaposition and remain under axial tension, sothat a coupling region is formed.
 8. A method of making apolarization-insensitive optical coupler as recited in claim 7, furthercomprising the step of rigidly encapsulating the nearly exposed cores ofthe first and second single-mode optical fibers while maintaining theparallel juxtaposition of the first and second longitudinal segments. 9.A method of making a polarization-insensitive optical coupler as recitedin claim 8, wherein said encapsulating step comprises rigidlyencapsulating the nearly exposed cores of the first and secondsingle-mode optical fibers in a quartz tube.
 10. A method of making apolarization-insensitive optical coupler as recited in claim 7, whereinsaid step (b) comprises the substeps of:(b1) wrapping portions of thefirst and second longitudinal segments with a material which will notsubstantially deform the nearly exposed cores of the first and secondsingle-mode optical fibers; (b2) bonding the nearly exposed cores of thefirst and second single-mode optical fibers together while the nearlyexposed cores of the first and second single-mode optical fibers arewrapped; and (b3) unwrapping the nearly exposed cores of the first andsecond single-mode optical fibers.
 11. A method of making a.[.polarization-insenstive.]. .Iadd.polarization-insensitive.Iaddend.optical coupler as recited in claim 10, wherein said substep(b2) comprises:gluing the nearly exposed cores of the first and secondsingle-mode optical fibers together with glue while the nearly exposedcores are wrapped; and curing the glue.
 12. A method of making apolarization-insensitive optical coupler as recited in claim 10, whereinsaid substep (b1) comprises wrapping portions of the first and secondlongitudinal segments with a thin plastic sheet.
 13. A method of makinga polarization-insensitive optical coupler comprising the steps of:(a)etching first and second longitudinal segments of first and secondsingle-mode optical fibers so that each fiber has an outer claddinghaving a low index of refraction relative to the index of refraction ofa nearly exposed core of each of the first and second single-modeoptical fibers; (b) maintaining the first and second single-mode opticalfibers in parallel juxtaposition with one another along a portion ofeach of the first and second longitudinal segments, said maintainingstep including the substeps of:(b1) wrapping portions of the first andsecond longitudinal segments with a material which will notsubstantially deform the nearly exposed cores of the first and secondsingle-mode optical fibers; (b2) bonding the nearly exposed cores of thefirst and second single-mode optical fibers together while the nearlyexposed cores of the first and second single-mode optical fibers arewrapped; and (b3) unwrapping the nearly exposed cores of the first andsecond single-mode optical fibers; and (c) fusing the paralleljuxtaposition portions of the first and second longitudinal segmentstogether to form a coupling region.
 14. A method of making apolarization-insensitive optical coupler as recited in claim 13, whereinsaid substep (b1) comprises wrapping portions of the first and secondlongitudinal segments with a thin plastic sheet.
 15. A method of makinga polarization-insensitive optical coupler as recited in claim 14,wherein said substep (b2) comprises:gluing the nearly exposed cores ofthe first and second single-mode optical fibers together with glue whilethe nearly exposed cores are wrapped; and curing the glue.
 16. A methodof making a polarization-insensitive optical coupler as recited in claim13, further comprising the step of rigidly encapsulating the nearlyexposed cores of the first and second single-mode optical fibers whilemaintaining the parallel juxtaposition of the first and secondlongitudinal segments.
 17. A method of making a.[.polarization-insenstive.]. .Iadd.polarization-insensitive.Iaddend.optical coupler as recited in claim 16, wherein saidencapsulating step comprises rigidly encapsulating the nearly exposedcores of the first and second single-mode optical fibers in a quartztube.
 18. A method of making a polarization-insensitive optical coupleras recited in claim 16, wherein:said maintaining step (b) furthercomprises the substep of placing the first and second longitudinalsegments under axial tenson; and said fusing step (c) comprises fusingthe parallel juxtaposition portions of the first and second longitudinalsegments together while the first and second longitudinal segments aremaintained in parallel juxtaposition and remain under axial tension..Iadd.
 19. A method of making a polarization-insensitive optical coupleras claimed in claim 7, wherein said step (b) comprises wrapping portionsof the first and second longitudinal segments with a material which willnot substantially deform the nearly exposed cores of the first andsecond single-mode optical fibers. .Iaddend. .Iadd.20. A method ofmaking a polarization-insensitive optical coupler as recited in claim 8,wherein said step of rigidly encapsulating the nearly exposed coresincludes the substeps of:placing a tube having ends and having a lengthsubstantially the same as the first and second longitudinal segmentsover the coupling region; gluing the ends of the tube to the first andsecond single-mode optical fibers with glue to suspend the fibers withinthe tube; and curing the glue. .Iaddend. .Iadd.21. Apolarization-insensitive optical coupler prepared by a method comprisingthe steps of:(a) etching first and second longitudinal segments of firstand second single-mode optical fibers so that each fiber has an outercladding having a low index of refraction relative to the index ofrefraction of a nearly exposed core of each of the first and secondsingle-mode optical fibers; (b) maintaining the first and secondsingle-mode optical fibers in parallel juxtaposition with one anotheralong a portion of each of the first and second longitudinal segments,said maintaining step comprising the substeps of:(b1) wrapping portionsof the first and second longitudinal segments with a material which willnot substantially deform the nearly exposed cores of the first andsecond single-mode optical fibers; (b2) gluing the nearly exposed coresof the first and second single-mode optical fibers together with gluewhile the nearly exposed cores are wrapped; (b3) curing the glue; and(b4) unwrapping the nearly exposed cores; (c) fusing together theportions of the first and second longitudinal segments maintained inparallel juxtaposition to form a coupling region; and (d) rigidlyencapsulating the nearly exposed cores of the first and secondsingle-mode optical fibers while maintaining the parallel juxtapositionof the first and second longitudinal segments, said step (d) includingthe substeps of:(d1) placing a tube having ends and having a lengthsubstantially the same as the first and second longitudinal segmentsover the coupling region; (d2) gluing the ends of the tube to the firstand second single-mode optical fibers with glue to suspend the fiberswithin the tube; and (d3) curing the glue. .Iaddend. .Iadd.22. Apolarization-insensitive optical coupler as recited in claim 21, whereinsaid step (a) comprises etching the first and second longitudinalsegments along a length in the range of 0.5 to 2 cm. .Iaddend. .Iadd.23.A polarization-insensitive optical coupler as recited in claim 21,wherein said step (a) includes removing an outer jacket of each of thefirst and second single-mode optical fibers along the length of thenearly exposed cores of the first and second single-mode optical fibers..Iaddend. .Iadd.24. A polarization-insensitive optical coupler asrecited in claim 21, wherein said step (a) includes rinsing the firstand second longitudinal segments of the first and second single-modeoptical fibers in hydrofluoric acid to remove any room temperaturevulcanizing rubber which may be surrounding the cladding of the firstand second single-mode optical fibers. .Iaddend. .Iadd.25. Apolarization-insensitive optical coupler as recited in claim 21, whereinsaid step (b) includes aligning the first and second single-mode opticalfibers in a holder having alignment grooves prior to said substep (b1)..Iaddend. .Iadd.26. A polarization-insensitive optical coupler preparedby a method comprising the steps of:(a) etching first and secondlongitudinal segments of first and second single-mode optical fibers sothat each fiber has an outer cladding having a low index of refractionrelative to the index of refraction of a nearly exposed core of each ofthe first and second single-mode optical fibers; (b) maintaining thefirst and second single-mode optical fibers in parallel juxtapositionwith one another along a portion of each of the first and secondlongitudinal segments while placing the first and second longitudinalsegments under axial tension; (c) fusing the parallel juxtapositionportions of the first and second longitudinal segments together whilethe first and second longitudinal segments are maintained in paralleljuxtaposition and remain under axial tension, so that a coupling regionis formed; and (d) rigidly encapsulating the nearly exposed cores of thefirst and second single-mode optical fibers in a quartz tube whilemaintaining the parallel-juxtaposition of the first and secondlongitudinal segments. .Iaddend. .Iadd.27. A polarization-insensitiveoptical coupler as recited in claim 26, wherein said step (b) comprisesthe substeps of:(b1) wrapping portions of the first and secondlongitudinal segments with a material which will not substantiallydeform the nearly exposed cores of the first and second single-modeoptical fibers; (b2) bonding the nearly exposed cores of the first andsecond single-mode optical fibers together while the nearly exposedcores of the first and second single-mode optical fibers are wrapped;and (b3) unwrapping the nearly exposed cores of the first and secondsingle-mode optical fibers. .Iaddend. .Iadd.28. Apolarization-insensitive optical coupler as recited in claim 27, whereinsaid substep (b2) comprises:gluing the nearly exposed cores of the firstand second single-mode optical fibers together with glue while thenearly exposed cores are wrapped; and curing the glue. .Iaddend..Iadd.29. A polarization-insensitive optical coupler as recited in claim27, wherein said substep (b1) comprises wrapping portions of the firstand second longitudinal segments with a thin plastic sheet. .Iaddend..Iadd.30. A polarization-insensitive optical coupler as recited in claim26, wherein said step (b) comprises wrapping portions of the first andsecond longitudinal segments with a material which will notsubstantially deform the nearly exposed cores of the first and secondsingle-mode optical fibers. .Iaddend. .Iadd.31. Apolarization-insensitive optical coupler prepared by a method comprisingthe steps of:(a) etching first and second longitudinal segments of firstand second single-mode optical fibers so that each fiber has an outercladding having a low index of refraction relative to the index ofrefraction of a nearly exposed core of each of the first and secondsingle-mode optical fibers; (b) maintaining the first and secondsingle-mode optical fibers in parallel juxtaposition with one anotheralong a portion of each of the first and second longitudinal segments,said maintaining step comprising the substeps of:(b1) wrapping portionsof the first and second longitudinal segments with a material which willnot substantially deform the nearly exposed cores of the first andsecond single-mode optical fibers; (b2) bonding the nearly exposed coresof the first and second single-mode optical fibers together while thenearly exposed cores of the first and second single-mode optical fibersare wrapped; and (b3) unwrapping the nearly exposed cores of the firstand second single-mode optical fibers; (c) fusing the paralleljuxtaposition portions of the first and second longitudinal segmentstogether to form a coupling region; and (d) rigidly encapsulating thenearly exposed cores of the first and second single-mode optical fibersin a quartz tube while maintaining the parallel juxtaposition of thefirst and second longitudinal segments. .Iaddend. .Iadd.32. Apolarization-insensitive optical coupler as recited in claim 31, whereinsaid substep (b1) comprises wrapping portions of the first and secondlongitudinal segments with a thin plastic sheet. .Iaddend. .Iadd.33. Apolarization-insensitive optical coupler as recited in claim 32, whereinsaid substep (b2) comprises:gluing the nearly exposed cores of the firstand second single-mode optical fibers together with glue while thenearly exposed cores are wrapped; and curing the glue. .Iaddend..Iadd.34. A polarization-insensitive optical coupler as recited in claim31, wherein:said maintaining step (b) further comprises the substep ofplacing the first and second longitudinal segments under axial tension;and said fusing step (c) comprises fusing the parallel juxtapositionportions of the first and second longitudinal segments together whilethe first and second longitudinal segments are maintained in paralleljuxtaposition and remain under axial tension. .Iaddend. .Iadd.35. Apolarization-insensitive optical coupler prepared by a method comprisingthe steps of: (a) etching first and second longitudinal segments offirst and second single-mode optical fibers so that each fiber has anouter cladding having a low index of refraction relative to the index ofrefraction of a nearly exposed core of each of the first and secondsingle-mode optical fibers; (b) maintaining the first and secondsingle-mode optical fibers in parallel juxtaposition with one anotheralong a portion of each of the first and second longitudinal segmentswhile placing the first and second longitudinal segments under axialtension; (c) fusing the parallel juxtaposition portions of the first andsecond longitudinal segments together while the first and secondlongitudinal segments are maintained in parallel juxtaposition andremain under axial tension, so that a coupling region is formed; and (d)rigidly encapsulating the nearly exposed cores of the first and secondsingle-mode optical fibers while maintaining the parallel juxtapositionof the first and second longitudinal segments, said step of rigidlyencapsulating the nearly exposed cores including the substeps of:(d1)placing a tube having ends and having a length substantially the same asthe first and second longitudinal segments over the coupling region;(d2) gluing the ends of the tube to the first and second single-modeoptical fibers with glue to suspend the fibers within the tube; and (d3)curing the glue. .Iaddend.